1. Field of the Invention
The present invention relates to a cordierite honeycomb structural body, particularly, to a highly airtight cordierite honeycomb structural body for a rotary accumulative type or a heat transfer type heat exchanger, which have excellent thermal shock resistance, airtightness and heat resistance, and a method of producing the same.
2. Related Art
Recently, the desire for materials having excellent heat resistance and thermal shock resistance has increased along with the improvement of industrial techniques for use thereof. The thermal shock resistance of ceramics is influenced not only by the coefficient or thermal expansion, heat transfer coefficient, strength, elastic moldulus; Poisson's ratio and the like in a material, but also by the size, shape and heating and cooling conditions in an article.
It is known that among these factors influencing the thermal shock resistance, particularly, the contribution of the coefficient of thermal expansion is large. Particularly, when the heat flow rate is large, the thermal shock resistance is significantly influenced only by the coefficient of thermal expansion. Therefore, there is a strong demand for the development of low thermal expansion materials having excellent thermal shock resistance.
Heretofore, cordierite ceramics have been known as relatively low thermal expansion materials. However, the densification of cordierite ceramics by sintering is difficult in general. Particularly, for the production of low thermal expansion cordierite ceramics having coefficients of thermal expansion of, for example, 2.0.times.10.sup.-6 /.degree.C. or less in a temperature range of from room temperature to 800.degree. C., batch composition of raw materials have been used wherein the amounts of impurities such as alkali, (calcium, potassium, sodium) and the like which function as fluxes during firing the batch composition are limited to very minor amounts, so that cordierite honeycomb structural bodies obtained by firing the batch compositions have very small amounts of a glassy phase and hence are very porous and are not dense.
Accordingly, when such a cordierite ceramic is, for example, formed into a honeycomb structure to be used in a rotary accumulative type heat exchanger, fluid leakage is generated due to high porosity from open pores which communicate between the heating fluid side of a partition wall and the heat recovery side of a partition wall, particularly pores communicating with the surface of the partition wall defining the penetration holes of the honeycomb structural body, with the result being that the heat exchanging efficiency and overall efficiency of heat exchanger system, decrease, which is a serious defect.
Thus, there has strongly been demanded a low thermal expansion and highly airtight cordierite honeycomb structural body having improved thermal shock resistance.
Heretofore, the low thermal expansion of cordierite ceramics has been publicly known. For instance, U.S. Pat. No. 3,885,977 discloses an oriented cordierite ceramic having a coefficient of thermal expansion of less than 11.times.10.sup.-7 /.degree.C.) in at least one direction in a temperature range of 25.degree.-1,000.degree. C., which is achieved by a planar orientation of clay platelets which occurs during the processing of stacked clay, or the use of platelet clay which also can result in such an orientation. However, that cordierite ceramic uses coarse talc having particle diameters of 10-20 .mu.m and clays of wide particle diameters of 0.1-10 .mu.m, and there is no disclosure concerning the pore structure of such materials.
Further, U.S. Pat. No. 3,950,175 discloses that a porous cordierite ceramic having open pores of a pore diameter of more than 10 .mu.m can be obtained in an amount of at least 20% by substituting silica or a silica alumina source material such as pyrophyllite, kyanite, quartz or fused silica in part or whole for talc or clay in the raw materials from which such ceramics are made. However, there is no description of the total pore volume of a pore diameter of not less than 5 .mu.m restricted to not more than 0.04 cc/g by using fine talc having an average particle diameter of not more than 7 .mu.m, whereby the airtight properties increase.
Furthermore, U.S. Pat. No. 4,280,845 discloses a positive correlation between the average particle diameter of talc particles and the average pore diameter of pores in cordierite ceramics. However, in order to restrict the coefficient of thermal expansion to not more than 1.0.times.10.sup.-6 /.degree.C., the talc particles should be made coarse, for example having an average particle diameter of 10-50 .mu.m. Therefore, it is difficult to conceive a low thermal expansion and highly airtight cordierite ceramic from that disclosure.
Finally, U.S. Pat. No. 4,489,774 discloses a rotary accumulative type heat exchanger consisting of highly airtight cordierite ceramics, wherein a filling material adhesively seals open pores on the surface of the partition walls defining the penetration holes of the honeycomb structural body, consisting essentially of cordierite ceramic of a porosity of 20-45%, but does not disclose a total pore volume of a pore diameter of not less than 5 .mu.m restricted to not more than 0.04 cc/g by using fine talc and kaolin particles having an average particle diameter of not more than 7 .mu.m and not more than 2 .mu.m respectively, whereby the airtight properties are improved. Additionally, the method described in U.S. Pat. No. 4,489,774 requires further steps for carrying, as a slip, a filling material on the fired cordierite honeycomb and refiring it, so that there are drawbacks such as the likehood of plugging the holes of the cells, and of high costs due to the more complex manufacturing steps.